This is a known, relatively rare bug (a similar problem can happen with the sight range and sight skill bonuses on equipment).
Does this happen if you remove the helmet?
The game may have messed up the resistances if you unequipped the dragon helmet; this happens more frequently if the skill is not otherwise known. There could also potentially be a problem if the sum of the levels that you learned yourself and the equipment bonus (usually) add up to 5 or more (at least one person said they only had an issue if the total was over 5, another said it happened to them at 5, as well).
The resistance values can be edited directly (see the Larian forum topic Some more hex offsets...
, and possibly Teleporting Anywhere, via hex editor
as a reference). If you have a problem with that, you can zip the data.000 file from your latest save folder and email it to firstname.lastname@example.org; zip the full save folder and I can double check it doesn't reoccur.
If you learn an additional level of Magic Barrier (if you have the character level and skill points to do so) or use iZakaroN's SaveEditor
to unlearn some levels of the skill, if applicable, that may at least help with the growth each time the save is loaded.
To keep this from happening again you can use DAD's Item Editor
to re-create your dragon helmet by editing another helmet in your inventory to have the same stats (you can add resistance bonuses directly rather than the skill bonus). You only need to wear the complete dragon armour set once to finish that quest, and have Bless cast on you (temporary effect); there are no set bonuses for keeping it together.
Alternately, make sure you end up with at least 1 but less than 3(?) levels in the passive Magic Barrier skill, or unlearn it completely.